PROTACs (or Proteolysis-Targeting Chimeras) are a novel, rapidly developing drug class that harnesses a cell’s native biology to inhibit rogue proteins. While traditional drugs bind a protein to inhibit it, PROTACs act as a linker between their target protein and an E3 ligase protein. Once the E3 ligase is brought in proximity, it attaches ubiquitin to the target. Because cells naturally use ubiquitination to designate proteins for degradation, the marked protein can be swiftly destroyed through the cell’s proteasome system.
Recently, the Crews group at Yale University published exciting new research on the use of PROTAC drugs to target cancer-causing oncoproteins. They targeted the KRAS protein, which is mutated in approximately 20% of cancers and is involved in cell proliferation (Bond et al.).
Specifically, they studied the prevalent G12C mutation, where an inert glycine residue has been swapped for a reactive cysteine. In cancers, this causes the KRASG12C protein to be overactive, driving cell proliferation (Bond et al.). Given the importance of this target, there are already several drugs in clinical trials that irreversibly bind to the KRASG12C cysteine, thereby inhibiting the protein.
The Crews group sought to improve on these drugs by creating a PROTAC. Because PROTACs link a target to an E3 ligase, they usually consist of a target-binding molecule attached to an E3 ligase-binding molecule. The authors chose one of the drugs currently in clinical trials as the target-binding region and attached it to a well-studied E3 ligase ligand through a linker section. This first PROTAC was bound to KRASG12C, but did not significantly degrade it. Since PROTACs are often very sensitive to their linker size, the researchers then tested a variety of shorter linkers. Their experimentation produced an improved PROTAC (dubbed LC-2) that, unlike its predecessor, was able to significantly degrade KRASG12C in a variety of cell lines (Bond et al.).
To confirm their discovery, the authors performed a series of competition experiments with a version of LC-2 that could not bind the E3 ligase, showing that LC-2 works through a PROTAC mechanism. Furthermore, the authors confirmed that degrading KRASG12C does in fact modify downstream signalling similarly to previous drugs, as expected (Bond et al.).
Perhaps most significantly, while traditional drugs were shown to eventually induce a compensatory increase in KRAS levels, the LC-2 PROTAC was less vulnerable to this resistance mechanism. LC-2 suppressed KRASG12C levels in all cell lines for up to 72 hours, and this suppression persisted past 72 hours in two out of three tested cell lines (Bond et al.).
The Crews lab’s research shows immense potential for creating more potent cancer treatments. However, they have not published any data on LC-2’s effect on in vivo models. Indeed, PROTACs face several challenges on their way out of clinical trials. Their large size can make getting the drug inside cells more difficult. Additionally, because LC-2 irreversibly binds to KRASG12C’s mutated cysteine, each PROTAC molecule can only degrade one KRASG12C protein. PROTACs that bind reversibly have the potential to degrade multiple copies of their target protein, and thus require a much lower dose. Despite the current challenges, PROTAC medicine has the potential to revolutionize how we treat cancer and other chronic diseases.
Works Cited:
Bond, M. J., Chu, L., Nalawansha, D. A., Li, K., & Crews, C. M. “Targeted Degradation of Oncogenic KRASG12C by VHL-recruiting PROTACs.” ACS Central Science, vol. 6, no. 8, 8 July 2020, doi:10.1021/acscentsci.0c00411.
Last Fact Checked on May 22nd, 2021.